Interferon (IFN) activity is marshaled early against the initial stages of viral infection, and IFN mediated responses play an essential role in host defense by directly inhibiting viral replication and by indirectly stimulating innate and adaptive immunity. Not surprisingly, IFN mediated signaling is a favored target of viral subterfuge. In this proposal we set out to explore the biophysical and functional attributes of two virally encoded decoy receptors that selectively neutralize the biological effects of type-l and type-Ill IFNs. Ectromelia virus encodes a decoy receptor that promiscuously binds and blocks the antiviral effects of all type-l IFNs in a species independent manner, with the notable exception of murine IFN-beta. The Yaba-like disease virus encodes a related decoy receptor that capably inhibits primate type-l (alpha and beta) as well as type-Ill (lamda) IFNs. Interestingly, neither of these viral proteins shares any significant sequence similarity with the distinct receptor components used for type-l or type-Ill IFN signal transduction. Our proposed studies aim to dissect the mechanism and consequences of selective decoy receptor mediated IFN inhibition using a diverse arsenal of experimental methods. In Aim 1 of the proposal we will determine the structural basis of IFN inhibition by viral decoy receptors using x-ray crystallography and quantitative protein interaction analysis. Results from these studies will be used in concert with yeast surface display directed evolution to develop variant decoy receptors with unique cytokine binding properties. In Aim 2 we will examine the cellular mechanisms of IFN decoy receptor function. The secreted poxvirus proteins tightly associate with cell surfaces through an unknown receptor interaction we propose to identify and investigate. We will also examine whether additional viruses encode IFN decoys. In Aim 3 we will explore the role of IFN sequestration in the mousepox pathogenesis model. Ectromelia recombinants will be generated that encode novel decoy receptors that selectively inhibit the actions of IFN-alpha, IFN-beta, or IFN-lambda at sites of viral infection in order to better understand the specific roles of these cytokines in anti-viral immunity.